The present invention relates generally to lighting, and more specifically to an arc discharge lamp, such as a ceramic metal halide lamp. This invention relates particularly to a means for applying conducting members to high pressure arc discharge lamps, e.g. sodium (HPS) arc tubes.
Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a xe2x80x9cdosexe2x80x9d emits a desired spectral energy distribution in response to being excited by the electric arc. However, arc discharge lamps, particularly those of a high pressure variety are often difficult to start.
Presently known metallic starting aids for discharge lamps which are composed of tungsten metal reduce the required electrical breakdown voltage for starting. Unfortunately, they may not survive an air firing step used for cleaning ceramic arc tubes. Moreover, the tungsten oxidizes and becomes useless as a starting aid. A similar oxidation problem prevents application of metallic starting aids on arc tubes intended for open air operation, such as ceramic or quartz mercury arc tubes for light projectors. Another problem with existing metallic starting aids is that tungsten is relatively expensive. Lower cost metals have higher vapor pressure, which, in the vacuum established within the outer envelope of HPS lamps, would evaporate to produce bulb darkening and lumen depreciation.
Previous HPS starting aids have been designed in the form of a wire or coiled ignition filament. The starting aid is positioned in contact with the outer surface of the arc tube and is connected to one electrical power lead of the lamp. When an arc is formed and the lamp begins to warm up, the electrical connection is either removed from the starting aid, or the starting aid is moved away from the arc tube, so as to prevent electric field accelerated sodium diffusion through the arc tube wall. Such sodium diffusion would adversely affect the lamp life. One drawback to this method for applying an external conducting member to HPS arc tubes is the cost and complexity of designing lamps with movable starting aids. Furthermore, in the absence of direct attachment to the lamp, the starting aid may sag away from the arc tube due to the high temperature of operation. If the starting aid is not in direct contact with the arc tube, anywhere along its length, the starting voltage increases. Additionally, these switches are typically attached to the lamp frame, resulting in heating by radiation, rather than by conduction. This results in variation of lamp performance depending on the wattage of different lamps.
It would therefore be desirable to find a means for applying an external conducting member to arc tubes which would overcome the above mentioned problems.
The present invention provides an arc discharge lamp comprising an arc tube including a starting aid. The starting aid comprises at least one metal cermet coating applied to the surface of the arc tube, or a metal layer applied to the surface of the arc tube with refractory glass disposed over the metal cermet or metal layer.
In another embodiment of the present invention, a method for forming an arc discharge lamp is provided. This method comprises binding a metal starting aid to an arc tube with refractory glass.
In a further embodiment of the present invention, the starting aid comprises a mixture of two or more metals.